This invention relates generally to high-voltage direct-current (HVDC) power transmission lines or systems, and pertains more particularly to composite filters for eliminating from the transmission system that harmonics that are produced by the converters utilized in changing the alternating current power to direct current power. High reliability of thyristor valves has allowed the adoption of 12-pulse conversion. Consequently, the characteristic harmonics are of the order 12n.+-.1 where n is a positive integer. The uncharacteristic harmonics associated with a balanced operation are quite small in magnitude. In view of the accelerating interest in HVDC power links, the requirement demanding proper filtering is expected to increase. The harmonics, if not properly filtered, can cause telephone interference, heating of electrical equipment and a ripple control malfunction.